1. Field of the Invention
The present invention relates to a transmission projection screen and a sheet for these screens.
2. Description of the Prior Art
Various measures have been taken for projection screens to diffuse light uniformly. For example, inorganic fine particles and organic polymer fine particles are contained in a resin sheet as a light diffusion agent, or special forms such as lens-like forms are given to the surface of a sheet to provide constant diffusion properties, which is widely seen nowadays.
To provide lens-like forms, a method of producing one by one in which an electron beam curable resin is poured into a metal mold having reverse forms to the lens-like forms and the obtained sheet is laminated with a plate-like molded product, and a continuous extrusion method in which a molten thermoplastic resin is shaped under a pressure with a roll having engraved reverse forms to the lens-like forms in a width direction (JP-A 9-11328) (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationsxe2x80x9d) are employed.
Since a high-quality image has been desired for a projection screen, a method for changing diffusion properties by making lenticular forms at the center portion lower and lenticular forms at the end portions higher (JP-A 7-134338, JP-A 11-237692, JP-A 2000-98499and U.S. Pat. No. 6,292,294) and a method for reducing the thickness of a screen to suppress a double image caused by the reflection of the screen itself and reduce cost are being developed and studied.
It is an object of the present invention to produce a high-quality transmission screen sheet for use as a light diffusing sheet for transmission projection screens at a low cost.
The inventors of the present invention have found that the production of a light diffusing sheet having a small thickness by the continuous extrusion method is the most effective in improving the quality and reducing the cost of a transmission screen sheet at the same time. However, they have found that a melt bank becomes nonuniform in shape along the extrusion width because the melt bank is sandwiched between a pair of rolls under pressure while forming the melt bank in the continuous extrusion system using shaping rolls, thereby making uniform shaping impossible.
That is, when the bank is large in shape, the resin to be sandwiched becomes large in quantity and pressure force grows, thereby increasing shaping properties whereas when the bank is small in shape, the resin to be sandwiched becomes small in quantity and pressure force shrinks, thereby reducing shaping properties. Further, as the size of the melt bank always changes slightly when the extrusion molding method is employed, it is difficult to continuously stably give lens-like forms with this method using a roll having a fine uneven pattern.
Meanwhile, when a roll having engraved grooves which differ in shape, height or pitch in the width direction and a roll having engraved grooves which are uniform in shape, height and pitch in the width direction are compared with each other, it is extremely difficult to continuously stably give lens-like forms with the roll having engraved grooves which differ in shape, height or pitch in the width direction.
Further, when the screen sheet is reduced in thickness, it is easily solidified by cooling while it is curved. In the step of separating the resin sheet from the shaping roll and making it flat, the resin sheet must be heated by a heater to remove its distortion. However, fine cracking may occur during molding with a groove in the lens as a starting point.
The inventors of the present invention have conducted intensive studies to solve these problems and have found that even when a roll having engraved grooves which change in shape non-stepwise is used, desired lens-like forms can be obtained stably regardless of the state of a melt bank by using a thermoplastic resin having a specific range of pressure dependence in a molten state. The present invention has been accomplished based on this finding.
That is, according to the present invention, there is provided a transmission screen sheet which is given a large number of lens-like forms on the surface like ridges and has a thickness of 0.1 to 2.0 mm, wherein (i) the sheet is formed from a thermoplastic resin whose inclination (W) obtained from the relationship between melt viscosity (Y) and load (X) satisfies the following expression (1) when the melt viscosity is in the range of 102 to 103 Paxc2x7s, and (ii) the lens-like forms change in height substantially non-stepwise from the center portion or a portion near the center portion to the end portions of the sheet in a direction perpendicular to the ridges:
2xc3x9710xe2x88x922xe2x89xa6Wxe2x89xa66xc3x9710xe2x88x922xe2x80x83xe2x80x83(1)
wherein W is represented by the following equation:
W=xe2x88x92(log Y/X)
wherein X is a load (MPa) and Y is the melt viscosity (Paxc2x7s) of the resin.